This invention relates to a method and apparatus for examining defects in or on sheet material, in particular bank notes, in particular for determining creases, tears, holes or dog-ears. The invention relates in addition to a bank note processing machine having such an apparatus.
The main area of use for the invention is to determine defects in bank notes. However, the invention is suitable for examining any sheet material, in particular for examining papers of value, whose quality can sink below a given standard through signs of wear.
Bank notes in circulation are generally tested for quality and authenticity after returning to a commercial and/or national bank. This test is normally done automatically in specially developed bank note processing machines. In case of a negative test result, the particular bank note is withdrawn from circulation. Quality is assessed with reference to so-called fitness criteria, which are determined for example with reference to soiling, tears, creases, holes, dog-ears and/or stiffness of the tested note in comparison to a new note.
U.S. Pat. No. 5,955,741 discloses a plurality of methods for assessing the fitness of bank notes with reference to their stiffness. Bank note paper contains long fibers that break through frequent use, so that notes lose their initial stiffness in the course of time. This structural change of the bank note paper is detected in order to indirectly infer the stiffness or derive a corresponding fitness criterion for the note. According to one of the methods proposed therein, the optical transmission or reflection properties of the note are detected. The note is thus irradiated with IR light (transmission measurement) or UV light (reflection measurement). The more IR light passes through the note or the more reflected UV light is scattered by the note surface, the poorer the quality of the note is to be rated.
The method proposed in U.S. Pat. No. 5,955,741 permits only a rough test of bank note properties, however. Large-area detection of reflected and transmitted radiation per-permits only statistical statements about defects in the paper. The contribution and size of individual defects is not determined.
The problem of the present invention is to propose an improved method and apparatus for examining defects in or on sheet material.
According to the invention, the sheet material is convexly curved and the defects located in the area of the convex curvature detected. Convex curvature of the bank note makes any defects more evident. Broken fiber ends protrude out of the paper, tears and holes are extended. Defects can thus be detected more easily.
Defects are preferably detected along an apex line or at individual points of an apex line. However, the inventive solution also quite generally provides convex curvatures of sheet material that have no apex line but a summit. Defects are accordingly detected in the area of said summit. Defects are preferably detected by means of an optical sensor. Optical sensors are inexpensive and available in numerous variants, so that they can be integrated into existing bank note processing machines at no great cost.
To permit the size and contribution of single defects to be individually detected, a preferred embodiment of the invention provides that an optical detector is disposed in an apex line plane of the convexly curved sheet material and directed toward the apex line so that the apex line of the convexly curved sheet material forms for the detector a kind of horizon above which defects of the bank note rise in silhouette. An apex line plane within the meaning of the invention is thus a plane tangential to the convex curvature, and the apex line within the meaning of the invention is defined by the tangent line between convex curvature and apex line plane. The convexly curved area of the sheet material will also be referred to as the apex in the following.
To permit optimal detection of the silhouette arising from the defects, it is advantageous if the apex of the curved sheet material is disposed against a light background. A uniformly light, homogeneous background can be obtained by means of a fluorescent lamp, a brightly illuminated surface, an LED row, or an LED array with a scattering medium disposed therebefore.
Precision of the test results can be improved if an optic is provided between the apex and the detector for imaging at least one point on the apex or apex line onto the detector. The imaging optic used may be for example a spherical, aspherical or cylindrical convergent lens or a self-focusing lens array (so-called Selfoc lenses).
A preferred detector includes a pixel array aligned parallel to the apex line. This permits adjacent areas of the apex to be separately detected and evaluated. The pixel array is preferably formed as a two-dimensional pixel array with pixels disposed in a uniform grid or as a one-dimensional pixel array with elongate pixels disposed perpendicular to the apex line. The individual pixels are formed as photosensitive elements, preferably as photodiodes or charge-coupled detector elements, so-called CCDs.
The silhouette caused by the defects is imaged on the pixels of the detector directed toward the apex line as a shadow, in particular against a light background when using an imaging optic. The more defects are present in the sheet material, the more elevations the silhouette has and accordingly more adjacent pixels of the detector are located in the shadow. The larger the defects are, the higher the silhouette is in the corresponding apex area and the more pixels disposed one above the other are located in the shadow. In the case of a one-dimensional detector array with elongate pixels disposed perpendicular to the apex line, the voltage per pixel is dependent on the height of the shadow falling on the pixel. This permits a measure of local density of defects to be derived from the number of unilluminated pixels, and a measure of size and/or an indication of the nature of the defects from the height of unilluminated pixels. For this purpose an accordingly formed evaluation device connected with the detector is provided.
A specific preferred embodiment of an apparatus for carrying out the test method provides that the convex curvature of sheet material is effected on a convexly curved component. This can preferably be a stationary linear element or a transport cylinder. Such components are readily present in bank note processing machines or can be added without any great effort.
Additionally, belts can be provided to ensure that sheet material rests reliably on the curvature of the convexly curved component. This reduces the danger of the apex line of convexly curved sheet material moving out of the focusing line.
Examination of sheet material can be effected during sheet transport so that the detector detects a silhouette changing in time which is evaluated by the evaluation device in real time. If defects fail to meet a given fitness criterion according to number and/or height, the corresponding bank note is withdrawn from circulation.
Apart from defects, the above-described system can also be used to detect and evaluate light reflexes due to strongly reflective areas, such as security threads, adhesive strips, kinegrams, etc., so that authenticity features of sheet material can be tested in addition or as an alternative to quality.